1. Field of the Invention
This invention relates to the field of making formed ceramic bodies. In one aspect, the present invention relates to a method of making a cast or jiggered ceramic body comprising casting a composition comprising a mullite-rich calcined kaolin obtained from a hydrous kaolin.
2. Background Art
Jiggering is a ceramic forming process wherein a shaping tool is used to shape the surface of a slug of clay that is formed onto the surface of a rotating mold. One surface of the jiggered product is defined by the mold surface, and the other by the shape produced but the shaping tool. Jiggering is particularly suited for the production of round and oval shapes such as plates and bowls.
Another ceramic forming process is slip casting. Slip casting is typically used in production of products having complex shapes and where plastic forming or semi-dry pressing are not possible. Thus, slip casting is applicable to the production of, for example, hollow tableware, figures and ornamental ware, and sanitary ware. For whiteware production, ‘jiggering’ can also be used to produce ware. Slip casting involves the use of a mold of appropriate shape into which a fluid suspension of a body can be poured and which mold progressively extracts some of the water until a solid layer is formed.
Two primary methods are typically employed for slip casting: drain casting and solid casting. In drain casting, a mold is filled with slip and casting takes place on one surface only. After a suitable time, during which the desired cast thickness is built up, the excess slip is poured off. The mold and cast are then partially dried, to allow mold release, after which the cast can be trimmed, cut or sponged. In solid casting, which is typically used for products having varying wall thicknesses, the mold is filled with slip and casting takes place on both surfaces. The removal of water generally means that the slip has to be topped up during the casting. For complex shapes, the mold can be constructed in several sections.
Rheological properties of casting slips can be important to the slip casting process. In order to make the casting process commercially acceptable, in standard methods, the rate of casting is as high as possible, which typically requires high solids content slips. High solids content slips are typically achieved by reducing the viscosity of the slip using deflocculants. Suitable slips are typically made in a moisture content range of about 27 to 35%.
Additionally, the texture of the cast can be important, which texture is affected by both the thixotropy and the fluidity. The permeability of the cast influences features such as drying. The body formulation can affect the density of the cast, which influences the casting rate.
The physical dewatering process depends upon several factors, including the solids concentration in the slip, the permeability of the cast, and the pore size distribution in the mold, which determines the suction and the permeability. The rate of casting relates to the permeability and porosity of the cast slip.
Previous methods of making cast ceramic bodies utilized hydrous clays. In one aspect, the present invention provides methods of forming a ceramic body using a mullite-rich calcined kaolin which yields significantly greater casting rates than previous casting methods. In another aspect, the present invention also provides products having much superior casting rates than the previous hydrous clays, for example hydrous kaolins and hydrous ball clays.
In yet another aspect of the present invention, mullite-rich calcined kaolin-containing clays are used to achieve compositions having surprisingly good casting rates, even when the kaolin comprises significant amounts of impurities. Thus, by the present method, one can even make cast ceramic bodies from kaolin clay that is often considered unusable for achieving fast casting, such as ball clay or other fine clays.